A Group III nitride semiconductor (also called a Group III nitride compound semiconductor or a GaN semiconductor) such as gallium nitride (GaN) has been used widely as a material for various semiconductor devices such as a laser diode (LD) and a light-emitting diode (LED). For example, the laser diode (LD) that emits blue light is applied to a high-density optical disc or a display, and a light-emitting diode (LED) that emits blue light is applied to a display or illumination. Moreover, an ultraviolet LD is expected to be applied to biotechnology and the like, and an ultraviolet LED is expected as an ultraviolet source of a fluorescent lamp.
Examples of a commonly-used method for producing a Group III nitride (e.g., GaN) crystal substrate include vapor deposition (vapor-phase epitaxial growth, i.e., Hydride Vapor Phase Epitaxy) and liquid deposition. Each deposition method has specific characteristics. The vapor deposition can be performed easily and thus is suitable for mass production. Examples of the vapor deposition include halide vapor phase epitaxy (Patent Document 1) and metalorganic chemical vapor deposition (MOCVD).